Field of the Invention
The invention relates to a method for detecting and correcting a saturated current profile of a current transformer, in particular for a medium-voltage or high-voltage system, with the aid of a pattern characteristic of a saturated current signal profile. The invention also relates to a device for carrying out the method.
In order to ensure the operation of a power supply system, a system disturbance, for example due to a short circuit, must be detected and located. Protection equipment or protective devices used for that purpose operate not only particularly quickly, but also selectively, so that the disturbance site is located by the protective device and only the disturbed item of apparatus is switched off. It follows therefrom that accurate knowledge of the response characteristic of a measuring transducer or current transformer assigned to the protective device is of particular importance for a correctly operating system protection. Since, in addition, in such a power supply system the primary current can be more than 100 A, in particular more than 400 A, in undisturbed operation, and can be more than 10 kA, in particular more than 40 kA, in the case of a short circuit, there is furthermore normally a need for a particularly large dynamic operating range of the measuring transducer or current transformer. However, the dynamic operating range is limited by the nonlinear magnetic coupling, due to core saturation occurring, between a primary winding and a secondary winding of the current transformer, with the result that in the case of a saturated current transformer the secondary current only partially represents an image of the primary current. That can lead to malfunctions of the protective device.
Consequently, current transformers with an anti-remanence air gap are frequently used at the highest-voltage level, or the current transformers that are used are overdimensioned in such a way that they are able themselves to transmit a fully asymmetrical short-circuit current in a non-saturated manner. However, the use of such an overdimensioned current transformer is particularly cost intensive and thus, for medium-voltage and high-voltage systems in particular, it is exceptionally uneconomical.
A more cost effective possibility is developing algorithms for calculating back from the saturated current profile to the unsaturated current profile of the current transformer. However, calculating accurately back to the saturated current is particularly intensive in terms of computer time. Thus, for example, International Patent Publication WO 93/13581 has disclosed a numerical method for reconstructing saturated transformer current signals in which a saturation and the end thereof of a secondary current measured in a time-discrete manner are determined with the aid of a transformer model for the unsaturated and the saturated transformer current as well as a model for the primary current.
Published European Patent Application 0 090 095 A1 has disclosed an evaluation method for the secondary current of a current transformer switched into a line of a power supply system on the primary side, in which a saturation signal is generated with the aid of a pattern characteristic of a saturated current signal profile. For that purpose, a Fourier analysis is performed on system-external frequencies, and a comparison between the filtered and the unfiltered secondary current is used to detect the saturation thereof. A comparatively quick detection of transformer saturation is achieved through the use of the method described in German Published, Non-Prosecuted Patent Application DE 39 38 154 A1, in which a possible deviation from the characteristic current waveform is detected by forming the space vector of the secondary transformer currents in the case of a blocked fundamental negative-sequence component.